Highlights

Empty Liquids and Equilibrium Gels in a Colloidal Clay

Sunday, 01 May 2011 08:05

CNR researchers (CNR-ISC and CNR-IPCF) in collaboration with University La Sapienza and ESRF (Grenoble) have observed a new kind of extremely light and stable gel in a suspension of colloidal clay. The so-called equilibrium gel, predicted 4 years ago by theoretical calculations by members of the same research team for a simplified model[1], could lead to improved drug-delivery systems and other novel microscopic devices.

A gel is a liquid that is rendered solid by a more or less rigid but disordered network of microscopic particles dispersed throughout its volume. These jellylike materials are extremely common and are used in everything from foods and pharmaceuticals to paints and cosmetics. However, many gels are made by “phase separating” a liquid suspension, which means cooling the liquid down until it splits into two distinct components, the more dense of which is the gel. Unfortunately, this is an unstable process that makes it difficult to control certain properties of the gel, including its density.

In the latest research, carried out over 7 years, Barbara Ruzicka (CNR-IPCF), Emanuela Zaccarelli (CNR-ISC) and colleagues have shown how an existing material—the synthetic clay Laponite, which is used as a thickener in many household products—can form a stable gel. The researchers suspended Laponite in water and studied how the structure of the suspension changes over time and how this evolution depends on the amount of clay present. At concentrations of up to 1% Laponite by weight, the initial fluid transformed into a gel after a few months, the researchers found. Then about 3 years later, it separated into two phases: one clay-rich and the other clay-poor. However, no such phase separation occurred at concentrations above 1%. Unlike at the lower concentrations, at which the arrangement of the clay particles was continually in flux, at concentrations above 1% the structure eventually stopped changing, indicating that the particles had locked into an extremely stable structure: the equilibrium gel.

This is made possible by the type of anisotropic interactions governing the clay disks behaviour. Indeed, typically colloidal particles interact in an isotropic way with all of their nearest neighbors when they form a gel. The relatively high density of particles needed to do this will not generally exist in the liquid state, but they can exist if the liquid undergoes phase separation[2]. Clay particles, in contrast, are disc-shaped and have an asymmetric charge distribution—a net negative charge on their faces and a net positive charge along their edges. So they do not interact with all of their nearest neighbors, but tend to form so-called T-bonds, in which faces and rims are connected in a sort of chain. Hence, the number of nearest neighbours with which they interact is limited, allowing them to form a gel at low density without the need for an intervening phase transition. This is a result of the limited valence of the particles in agreement with theoretical and numerical predictions for patchy colloidal spheres[3].

It is blue or green? How humans name categories.

Tuesday, 03 February 2009 11:50

An ancient welsh would probably answer "glas", the word which refers to blue and certain shades of green in traditional Welsh. Even today a Japanese taxi driver can call "ao" the color of the green traffic light, a word which is usually translated as "blue". Actually several languages do not distinguish between green and blu (read more).

Although colors in the rainbow change continuously we have sharp names for the colors. Thus blue refers to a region or "category" of shades but the borders of this category actually fluctuate, i.e. they are different among different languages, different persons speaking the same language, and even different moments for the same person. This randomness of words-defined categories is better appreciated when the categories belong to a "space" which does not present clear divisions, interruptions, discontinuities: the case of colours is, for this purpose, one of the most representative. Our perception of the light spectrum is mostly continuous (the sum of all cones in human retina produce a quite smooth absorption curve across the visible spectrum) and this makes puzzling the way each culture separates such a continuum in different regions associated to colour-terms. Are these categories determined by the way we perceive colors? In other words are they determined by human biology and thus are universal among cultures or they are influenced by culture?

A seminal empirical study performed in the '60s of the past century (and improved along years), by interviewing individuals from many different languages across the world [1,2], provided fundamental results in the study of color terms: a series of "invariant" emerged from this study, i.e. properties shared by all languages. In particular a basic intuition was confirmed: that the color terms in a language are a few, typically less than 10, even much less in some small cultures still not reached by modern technology. Universal properties of basic color terms have been offered as an example of the so-called linguistic "universals", considered by many as an evidence for the strong influence of biology on the evolution of languages.

A recent wave of linguistic studies, based on computer-simulated multi-agents models [3-4], has turned back to (re)consider the contribution of "cultural activity" (interaction among agents) for language evolution: in this framework, biological constraints play the role of "game rules", the same for all agents in all languages/cultures. Notwithstanding the universality of such rules, a certain degree of variation (fluctuations) is observed across different agents and different populations, thanks to the statistical nature of the collective dynamics: a complicate and not entirely predictable behavior emerge from letting the agents play with their rule for a long time. In a recent study published on PNAS, Andrea Puglisi and Vittorio Loreto of ISC-Sapienza together with Andrea Baronchelli of the Universitat Politècnica at Barcelona, provided a new model which is able to reproduce the puzzling phenomenon discussed above, present in human languages: our dictionary contains less than 10 colour terms, while our eyes can perceive hundreds (if not thousands) of different wavelengths. The process of categorization appears as a kind of coarsening process (on the perceptual space) which slows down and apparently "arrests" in a final configuration, a pattern of few colour terms associated to a very large perceptual space. The researchers say that this slowing down resembles, in some aspects, the phenomenology of undercooled molecular liquids which arrest in a glassy phase providing another example of cross fertilization we have seen before in the science of complexity.

More recently, the same authors have compared the results of their agent-based model with some statistical features of real data from the World Color Survey, discovering an impressive and unexpected quantitative agreement. Such an agreement is achieved by adding to the original model a minimal bias of biological origin, i.e. the so-called "jnd" (just noticeable difference) curve, which gives the average human spectral resolution at a given light wavelength. The authors have verified that correlations between colors patterns in different languages produced by the model are very close to those observed in human languages. The researchers conclude with a twofold consideration: a) "jnd" is perhaps the minimal biological constraint inducing similarities among human languages (in the color dictionary); b) the proposed agent-based model is not far from reproducing human language dynamics.

How to put biology and physics on the same scale

Tuesday, 13 January 2009 16:40

Like it or not, in the last decades science departments and funding agencies have been increasingly using citation metrics to allocate grants and positions. But serious problems arise when comparing citation performance across different disciplines. In some fields, like mathematics, citation numbers are generally much higher than in others, like biomedical research. For example, the highest 2006 impact factor for a mathematical journal was 2.55, twenty times smaller than the impact factor of theNew England Journal of Medicine. Physics lies more or less half way between these two extremes. A similar problem occurs when comparing papers published in different years. Older papers are on average more cited, not because they are better than more recent ones, but simply because they had more time to accumulate citations.

How to compare fairly citations of papers in different disciplines or published in different years?

Claudio Castellano from ISC-Sapienza and colleagues have found a way to put citation numbers of papers in different disciplines and different years on a universal scale. This allows to show, for example, that a paper published in 1999 in aerospace engineering cited 20 times had more impact than an article published the same year in developmental biology.This method opens the way towards an unbiased evaluation of performance also of individual scientists, research groups or departments.

When repulssion helps superconducticity

Friday, 09 January 2009 15:44

Our researchers have found a mechanism by which Coulomb repulsion, rather than hamper superconductivity, enhances it. Since superconductivity requires electron pairing, Coulomb repulsion is usually considered detrimental for superconductivity still most high Tc materials are considered also strongly correlated. Correlation enhanced superconductivity appears when a strong Coulomb interaction dresses the particles in a symmetry channel different from the one where pairing occurs. This research has been published in Science, Phys. Rev. Lett. and a work is in press in Reviews of Modern Physics (colloquium).